Various conventional analog to digital (A/D) converters provide a digital signal that is representative of the instantaneous magnitude of an applied voltage. In one type of A/D converter, for example, the input voltage is applied to a voltage to frequency converter, and the frequency of the output signal is measured by counting output pulses over a fixed time interval. Examples of A/D converters of this and other types are described in Chapter 14 of Wobschall, Darold, Circuit Design for Electronic Instrumentation, McGraw Hill, 1979.
In another type of A/D converter, termed a "ramp converter" the input analog waveform is applied to an integrator. Following initialization of the integrator, the integrator generates a ramp voltage having a slope that is a function of the input voltage magnitude as well as of the time constant of the integrator. The instantaneous magnitude of the ramp voltage is compared in a comparator with a reference voltage. The output of the comparator controls a gate which supplies pulses generated by a clock source to an output counter that provides a pulse count proportional to input voltage magnitude. Because the slope of the ramp is a function of the size of the capacitance as well as of the magnitude of the input voltage, single ramp converters require a precision, high quality integration capacitor, such as one made of polystyrene or polypropolene which is expensive. Even with such precision components, however, the accuracy of single ramp converters tends to be sufficient for critical applications such as in laboratory volt meters.
Dual ramp converters have conversion accuracies that are higher than those possible with single ramp converters since the conversion count is independent of the time const. and clock freq. Conversion takes place in two phases, the first phase wherein the input voltage being measured is applied to the integrator to up-ramp and the second phase wherein a reference voltage of polarity opposite that of the input voltage is applied to the integrator to down-ramp. The time required for the integrator output to return to zero or other predetermined reference during ramp-down is monitored by a comparator which controls pulse count in an accumulator. The advantage of the dual slope converter over the single slope converter is that accuracy depends substantially only on the accuracy of the reference voltage source and, as mentioned above, not on the accuracy of the integrating capacitor.
Implicit in the dual slope strategy, however, is that the output accumulator or counter must be precisely controlled to accumulate pulses only during the ramp-down interval. Accordingly, one object of the present invention is to provide an analog to digital converter that is highly accurate and does not require high stability components or complex circuitry.
Another object is to provide a dual ramp analog to digital converter, wherein the ramp-down interval is precisely determined to control output pulse count.
Conventional dual ramp converters, such as the type shown in Ammann U.S. Pat. No. 3,316,547, require an analog signal source for supplying the voltage to be measured to one input of an integrator during ramp-up and a reference voltage source to be applied to the integrator during ramp-down. Because the reference voltage must have a polarity opposite that of the input voltage, as explained above, the basic dual ramp converter is operative only with single polarity input voltages. To be operative with dual polarity sources, the polarity of the input signal at the end of the ramp-up interval must be measured and the polarity of the reference adjusted during the ramp-down interval to be opposite that of the input voltage. To provide reference voltage polarity reversal, a circuit known as a "flying capacitor" has previously been developed, wherein, during an initialization interval, a reference or flying capacitor is charged from the supply voltage. During ramp-up, the flying capacitor is disconnected from the supply source and allowed to "float" and thereafter, during the ramp-down interval, the capacitor is connected, with polarity opposite the polarity input voltage, to the integrator.
To further complicate circuit requirements, in precision instrumentation, particularly when such instrumentation is interfaced with other equipment, the analog circuitry including the integrator and comparator is electrically isolated from the digital circuitry including the pulse counter or accumulator and digital display. In prior art of which I am aware, the generation of start and stop pulses to be applied to control pulse accumulation in an external counter during the ramp-down interval has been imprecise in dual slope converters of this kind. These converters contain an internal counter used for timing or controlling switching between the integrator inputs and to accumulate counts during ramp-down, and thereafter to store count accumulation in registers for decoding. An external counter electrically isolated from the converter makes the pulse count derived by the converter available to external equipment, and to control data format. In converters incorporated within large scale integrated circuits containing both analog and digital signal processing, intermediate signals are not available to control pulse accumulation in an external counter. Even an error on the order of one or two percent is intolerable in precision instrumentation, such as laboratory volt meters. It is therefore necessary to derive external counter start and stop signals in integrated circuit type, dual slope A/D converters where such signals are not directly available.
Another object of the invention, therefore, is to provide precise control of the pulse count stored externally in a counter during ramp-down in dual slope comparators of the dual polarity type.
Another object is to provide precise pulse count accumulation during ramp-down in dual slope accumulators of the type using a "flying capacitor" to establish a dual polarity reference from a single polarity voltage source.
Another object is to provide precise pulse count control during ramp-down in a dual ramp comparator of the type wherein a dual polarity reference voltage is obtained from a single polarity source using a "flying capacitor" and wherein the analog and digital circuits are electrically isolated from each other.